Electrical discharge treatment of tetrafluoroethylene/hexafluoropropylene copolymer in acetone

ABSTRACT

TREATMENT OF TETRAFLUOROETHYLENE/HEXAFLUOROPROPYLENE COPOLYMER SURFACE BY ELECTRICAL DISCHARGE IN AN ATMOSPHERE CONTAINING 3 TO 40% BY VOLUME OF ACETONE TO RENDER THE SURFACE MORE ADHERABLE TO ANOTHER MATERIAL.

United States Patent 3,676,181 ENT ELECTRICAL DISCHARGE TREATM OFTETRAFLUOROETHYLENE/HEXA- FLUOROPROPYLENE COPOLYMER IN US. Cl. 117-47 AABSTRACT OF THE DISCLOSURE Treatment oftetrafluoroethylene/hexafluoropropylene copolymer surface by electricaldischarge In an atmosphere containing 3 to 40% by volume of acetone torender the surface more adherable to another material.

This invention relates to a process for the treatment of atetrafiuoroethylene/hexafluoropropylene surface to render it moreadherable to another surface. More particularly this invention relatesto the treatment of a fluorocarbon surface by electrical discharge in anatmosphere containing between 3 and 40% acetone.

It is known in the art that fluorocarbon surfaces are highly inertchemically. Because of this it is dlfficult to obtain good adhesionbetween a fluorocarbon surface and another material. One solutionsuggested by the prior art to improve the adherability of fluorocarbonis to treat it with a corona discharge. By means of such treatment it ispossible to obtain a fluorocarbon surface that Wlll adhere to anothermaterial such as a polyimide layer. Such a process is fully disclosed inthe Anderson et al. lat. 3,352,714.

It is also known to improve the adherabihty of a flllOI'O carbon surfaceby electrical discharge treatment of the surface in an atmospherecontaining less than 5% by volume of an organic compound, such asglycidyl methacrylate. See US. Pats. 3,296,011 and 3,274,089.

The process of this invention for improving the adherability of afluorocarbon surface to another matenal comprises treating thefluorocarbon surface with an electrical discharge in an atmospherecontaining between 3 and 40% by volume of acetone. Preferably, theatmosphere contains between 15 and 30% by volume of the acetone, whilethe most preferred concentration is about 20% by volume. The atmospherealso contains a carrier gas that is substantially inert under theconditions of treatment. Such gases include nitrogen, helium, argon,carbon dioxide and the like as well as mixtures thereof which containless than 600 p.p.m. by volume oxygen. The preferred oxygen content isless than 100 p.p.m. by volume.

A suitable apparatus for carrying out the process of the presentinvention is fully described in US. Pat. 3,274,089. A continuousself-supporting fluorocarbon film is passed continuously between a setof spaced electrodes consisting of a rotating metal roll which isconnected electrically to ground, and one or more stationary hollowmetal tubes disposed parallel to the longitudinal axis of the roll andspaced a distance of from 0.03 to 0.125 of an inch from the surfacethereof. The tubes are each connected electrically to a suitable powersource which supplies an alternating (or pulsating direct) current ofthe required intensity at the required voltage and frequency. Theacetone containing atmosphere, i.e., acetone admixed with a suitablecarrier gas, is fed continuously to the hollow interior of the electrodetubes through distributor ducts and issues from the tubes, throughsuitable openings therein, at the gap betwen each tube and the roll. Theelectrical discharge takes place in the atmosphere containing theacetone. The atmosphere may also be introduced into the reaction zonethrough one or more tubes separate from the electrode ice assembly. Theassembly just described is suitably enclosed in a chamber, held atsubstantially atmospheric pressure and provided with the necessaryopenings to facilitate maintenance of the atmosphere of carrier gas andacetone therein and to permit controlled exhaust of the vaporstherefrom. The treated film may be passed through a heating zone and/ora coating apparatus whereby to further condition the surface of the filmto enhance the permanency of the effect of the treatment.

In carrying out the surface treatment of this invention the potentialdifference between the electrodes may vary from low voltages in theorder of 1000 volts up to pulsating voltages of 100,000 and above. Ingeneral, however, it is preferred to maintain the voltage in excess of2000- 3000 volts. Frequencies from 350 cycles per second up to 500,000cycles per second and above can be used. Frequencies in the range ofabout 6,000 to 15,000 cycles are preferred in order to obtain effectivetreatment at commercially acceptable exposure times. While the currentto the electrodes may range up to 360 R.M.S. (root mean square)milliamperes per square inch of electrode or more, for optimum results arange of from 20 R.M.S. milliamperes per square inch of electrode to 230R.M.S. milliamperes per square inch of electrode is preferred. Power tothe electrodes may range from 10 watts per lineal inch of the electrodelength to watts per lineal inch of the electrode length.

The electrodes are preferably spaced from about .03 inch to about 0.1-25inch. Useful results can be obtained when the electrode gap is as low as0.015 inch to as much as 0.25 inch, provided suitable adjustments insuch features as amount of current, electrode dimension and exposuretime are made. Time of exposure to the electrical discharge treatment isnot especially critical and effective treatments are realized atexposure times as short as 0.01 second and no adverse effects are notedat times as long as 60 seconds. Preferably the exposure time should notbe less than 0.1 second. For economic reasons, exposure times as shortas possible consistent with effective treatment would normally beemployed.

In order to achieve the desired increase in adhesion, thetetrailuoroethylene/hexafluoropropylene surface should be subjected tobetween 0.15 and 2.5 watt hrs. per square foot of sheet surface treated.

Flow of the carrier gas/ acetone mixture to the electrodes may be as lowas one-twelfth cubic foot per foot of electrode per minute up to 1.67cubic feet per foot of electrode per minute. Higher flow rates can beused though economic considerations would dictate against use of amountsexceeding those required to produce the desired effects.

The carrier gas/acetone mixture may be obtained by bubbling the carriergas through acetone. Carrier gas that is approximately saturated withacetone will be obtained using this method. In the case where nitrogenis used as the carrier gas, at room temperature (20 C.) the amount ofacetone is about 20% by volume. The amount of acetone in the atmospherecan be regulated by regulating the temperature of the carrier gas, orthe amount of acetone regulated by feeding into the carrier gas/acetonemixture additional carrier gas.

The tetrafiuoroethylene/hexafluoropropylene copolymer films may betreated either on one surface or on both surfaces, depending on the useto which the films are to be put after treatment.

Tetrafluoroethylene hexafluoropropylene copolymer films treated inaccordance with the present invention form bonds that are more durablethan those formed in previously used treatments, i.e., treatments inatmospheres containing glycidyl methacrylate, when exposed to moistureor when exposed to temperatures of about 200 C. Furthermore, the surfaceis more resistant to loss of adherability caused by abrasion or exposureto ultraviolet light.

This film has the further advantage that it does not adhere tightly toitself, and thus may be wound in rolls without an interleaf, or sheetsof the film may be stacked without severe blocking.

EXAMPLE 1 A sample of mil thick tetrafiuoroethylene/hexafluoro propylenecopolymer (wt. ratio 85/15) film of the type described by US. Pat.2,946,763, was placed between metallic electrode plates 2 /2 inches indiameter and spaced 50 mils apart, positioned inside a gas-tightenclosure. After purging the enclosure with nitrogen until its oxygenconcentration had dropped to 50 p.p.m. by volume, acetone vapor wasintroduced into the space between electrodes. This was done by passing astream of nitrogen at about 20 C. through a gas washing bottle filledwith liquid acetone, and then directing the resulting mixture ofnitrogen/acetone to a flat distributor nozzle located just alongside theelectrodes. An electrical discharge was then established across theelectrodes by supplying 6,000 volts at 60 Hz. from an oscillator andpower amplifier. A current flow of 165 a. was obtained. After 30 secondsthe power was cut off and the treated sample removed.

Adherability of the treated surface was measured by laminating the filmto an aluminum strip 6 inches long by 1 inch wide by 35 mils thick. Thealuminum was first coated with Du Ponts Adhesive No. 6840 (an acryliccopolymer adhesive in which the copolymer is dissolved in a mixture ofalcohol and aromatic hydrocarbon) and permitted to air dry overnight. Afilm sample cut to the same 1" wide x 6" long dimensions was then placedsuch that the treated surface of the film was in contact with theadhesive coated aluminum strip and then laminated at 20 p.s.i. in aSentinel Heat Sealer, Model l2AS, for 60 seconds at 375 F. Only the lastinch of length was pressed and cured together, leaving the remaininglengths unbonded so they could be gripped in the jaws of an Instrontensile tester and pulled apart in 180 peel to measure bond strength. Inthis test, the acetone treated film tore, indicating that bond strengthexceeded film tensile strength.

The experiment was repeated with glycidyl methacrylate, approximately0.05% by volume in nitrogen. Film peeled away from the test laminate at6.7 lbs./ inch.

EXAMPLE 2 TABLE Bond strength Film in 180 thickness, peel, mils lbs/in.

20 10 8. 3 10 16. 2 Methyl alcohol. 5 11. 1 Methyl methacrylate 5 8. 5Xylene. 5 2 10 1 Where film tore rather than delamlnated from thealuminum strip, bond strengths are shown as greater film tensilestrength values.

2 Dropped to 5.6 lbs/in. after 2 weeks aging at room temperature.

EXAMPLE 3 A device was constructed which comprised a stationary barelectrode, A" wide and 6" long, positioned parallel to and above arotatable, electrically grounded drum upon which film samples could bemounted. The whole assembly was enclosed in a gas-tight transparent caseinside which '4 an essentially oxygen-free atmosphere could bemaintained by purging with nitrogen. A perforated-tube gas distributorwas provided for directing the organic vapor and nitrogen mixture intothe gap between electrode and drum.

After first covering the drum with a dielectric bufier material toprevent any possible bum-through of the film being treated, an 8 inch x14 inch sample of a tetrafluoroethylene/hexafluoropropylene copolymer(wt. ratio 15) film was wrapped tightly around the drum and clamped inposition. With the enclosure replaced, nitrogen was purged through untilan oxygen analyzer showed the interior to have only 50 p.p.m. by volumeof 0 present. The drive motor was then started and the drum continuallyrotated at a surface speed of 8.3 feet per minute. Nitrogen, regulatedat a fiow rate of 3,500 ml./min., was bubbled through liquid acetone tobecome essentially saturated with acetone vapor and this mixture was fedto the gas distributor and into the gap between electrode and drum.Power was supplied to the electrode to establish an electricaldischarge, which was maintained for two complete revolutions of the drumand then shut off. The power source consisted of an oscillator whichgenerated an alternating current at 10,000 Hz., coupled to a 400 VApower amplifier and a 30:1 step-up transformer. Voltage at the electrodewas kept at 6,000 v.

The treated film was removed from the drum and cut into 1 inch x 6 inchstrips for testing. Adherability was measured by the method described inExample 1. To determine resistance of the adherable surface todegradation by moisture, several of the samples were first immersed inboiling water for 16 hours and then tested.

Similar samples were prepared in the same manner except that glycidylmethacrylate was used in place of acetone.

Results are compared below:

Peel strength, lbs./in.

The objective of this experiment was to compare ability of the treatedsurfaces to withstand abrasion Without extensive loss of adherability.This property is important to manufacturers who use such film for makingelectrical printed circuits.

Samples of both acetone and glycidyl methacrylate treated film wereprepared by the method described in Example 3. They were then abraded bya weighted bristle brush in a Gardner Straight-Line Wash Tester, withsuccesive samples receiving an increased number of brush strokes.Adherability of each sample was then measured by the peel test describedin Example 1. Results are tabulated below:

Peel strength, lbs/in.

Additional samples were prepared in the same way and tested forresistance to ultraviolet light by exposing them in a Fadeometer for 200hours. Adherability was measured after exposure. Results are:

Peel strength, lbs./iu.

GMA Acetone treated treated AS made 6. 64 l 20. After 200 hrs. inFadeometer 4. 64 11. 16

1 Sample tore.

Peel strength, lbsJin.

GMA Acetone treated treated As made 1 9. 3 1 9. 5 After 8 hrs. at 225 C.in alr 8. 3 1 9. 0 After 1 day at 225 C. in alr- 3. 6 1 8. 7 After 2days at 225 C. in air.-- 0 7. 4 After 4 days at 225 C. in air 0 3. 4After 8 hrs. at 225 0.111 N2..-" 3. 0 1 9.0 After 1 day at 225 C. inN2.... 1.0 1 9. 0 After 2 days at 226 C. in N2- 0 l 8.7 Aiter4daysat 225C.1I1Nz--- 0 8.7

4 Film tore.

What is claimed is:

1. In a process for the production of atetrafiuoroethylene/hexafiuoropropylene copolymer film that has asurface that is adherable to another material which comprises subjectingsaid fluorocarbon film to an electric discharge between spacedelectrodes in a gaseous atmosphere, the improvement which comprisesemploying as the atmosphere a mixture containing substantially inertcarrier gas and between 15 and 30% by volume acetone.

2. The process of claim 1 in which the film is subjected to between 0.15and 2.5 watt hours of electrical energy per square foot of sheetingsurface treated.

3. The process of claim 1 in which the carrier gas is nitrogen.

4. The process of claim 1 in which the film is treated on both surfaces.

5. The process of claim 3 in which the nitrogen is at about 20 C.

References Cited UNITED STATES PATENTS 3,415,683 12/1968 Coifman et al204-168 3,507,763 4/1970 McBride 204-169 3,255,099 6/1966 Wolinski204----169 3,274,089 9/ 1966 Wolinski 204-169 3,274,090 9/1966 Amborski204168 3,274,091 9/1966 Amborski 204-169 3,275,540 9/1966 McBride204-169 3,296,011 1/ 1967 McBride 204-169 OTHER REFERENCES NASADevelops. Technique for Plating Metals on Metals or FluorocarbonPolymers, Products Finishing, pp. 77-74, March 1966.

MURRAY KATZ, Primary Examiner W. R. TRENOR, Assistant Examiner U.S. Cl.X.R.

117-93.1 CD, 119, 138.8 UF; 204-168, 169

